AbstractThe mononuclear cell content and mononuclear cell function of human solid tumors were determined. Twenty‐eight primary, and 17 metastatic solid tumors, including 6 lymph node metastases, were obtained from 45 patients. Cells were released from the tumors by mincing, scraping, and sieving. Analysis of the cellular content was determined by light microscopy, and tumor lymphocyte function was determined by stimulation of blastogenesis with PHA, PWM, Con‐A, MLC, and SLO. A median of 3.3 × 107tumor cells with 12.5% viability was obtained per gram of tumor, and a median of 7.8 × 106lymphocytes with a viability of 96% and 0.35 × 106macrophages with a viability of 85% was also obtained per gram of tumor. Significant lymphocyte responses to various mitogens and antigens were obtained in 31–40% of the tumor lymphocyte preparations. From 9.4% to 23% had vigorous responses with a stimulation index greater than 10. Primary tumors contained significantly more lymphocytes than metastatic tumors; thus, the percent lymphocytes in primary tumors was 13.5% compared to 5% for metastatic tumors. In contrast, some of the lymphocyte responses were significantly greater for metastatic tumors than primary tumors. These data suggest that the direct study of lymphocytes infiltrating primary and metastatic tumors may be a useful approach to understanding tumor immunit